Occasional harvesting of crop residue from fields will have little impact on soil productivity, but continuous removal – either through burning or other means – could lead to less productive soils, according to 20 years' worth of research done by Kansas State University.

"When crop residues are harvested, organic biomass and plant nutrients are leaving the field," says Keith Janssen, agronomist-in-charge of the K-State Research and Extension East Central Experiment Field near Ottawa, the site of the study. "Continual crop residue harvesting reduces soil pH, exchangeable potassium (K), and organic matter and that will eventually lead to less productive soils."

The study involved a soybean-wheat-sorghum/corn crop rotation. Three residue treatments were used, including annual harvesting of crop residue; normal residue incorporated; and two-times the normal residue incorporated. Four fertilizer levels were used: zero, low, normal and high for each crop.

The big differences among the residue treatments were found in soil properties, Janssen says. Soil pH, exchangeable K, and soil organic matter decreased when the crop residue was harvested. Soil pH only dropped by 0.1%, but exchangeable K in the soil plummeted by about 20% and the organic matter level dropped by 9%. However, the two-times residue treatment increased both the exchangeable K and organic matter and the incremental increase infertilizer treatments progressively lowered soil pH.

"Why the changes in soil properties with crop residue harvesting?" Janssen asks. "Crop residues contain high levels of K, so when crop residues are harvested, as with silage, there's a drain on soil K levels. Also, the soil organic matter declines because the crop residue removed is the main carbon source that becomes the soil organic matter.

"As one might suspect, there were large differences in grain and residue yields among the fertilizer treatments with the normal and high rates resulting in the highest yields," Janssen says.

There were few differences, however, in grain and residue yields among the residue treatments, but that could change with longer-term, repeated harvesting of crop residues.